Solar Cell Chapter 6: Design of Silicon Solar Cells

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Solar CellChapter 6 Design of Silicon Solar
Cells

• Nji Raden Poespawati
• Department of Electrical Engineering
• Faculty of Engineering
• University of Indonesia

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Contents

• 6.1. Optical Properties
• 6.2. Reducing Recombination
• 6.3. Top Contact Design
• 6.4. Solar Cell Structure

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Optical Properties
Basic Solar Cell Design Solar cell design
involves specifying the parameters of a solar
cell structure in order to maximize efficiency,
given a certain set of constraints. Fig. 1
shows Evolution of silicon solar cell efficiency.

4
Optical Properties(continued)

• In designing such single junction solar cells,
  the principles for maximizing cell efficiency
  are
• increasing the amount of light collected by the
  cell that is turned into carriers
• increasing the collection of light-generated
  carriers by the p-n junction
• minimizing the forward bias dark current
• extracting the current from the cell without
  resistive losses.

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Optical Properties(continued)

• Optical Losses
• Optical losses chiefly effect the power from a
  solar cell by lowering the short-circuit current.
  
• Sources of optical loss in a solar cell is
  illustrated in Figure 2.
• There are a number of ways to reduce the optical
  losses
• Top contact coverage of the cell surface can be
  minimized (although this may result in increased
  series resistance)
• Anti-reflection coatings can be used on the top
  surface of the cell.
• Reflection can be reduced by surface texturing.
• The solar cell can be made thicker to increase
  absorption
• The optical path length in the solar cell may be
  increased by a combination of surface texturing
  and light trapping.

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Optical Properties(continued)

• Anti-Reflection Coatings
• Anti-reflection coatings on solar cells are
  similar to those used on other optical equipment
  such as camera lenses.
• The minimum reflection is calculated by

(6.1)
Where n1 a refractive index of transparent
material (ARC) l0 a free-space wavelength and
d1 the thickness n0 a refractive index of
glass or air n2 a refractive index of
semiconductor
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Optical Properties(continued)

• Figure 3 illustrates use of a quarter wavelength
  anti-reflection coating to counter surface
  reflection.
• For photovoltaic applications, the refractive
  index, and thickness are chosen in order to
  minimize reflection for a wavelength of 0.6mm.
• Comparison of surface reflection from a silicon
  solar cell, with and without a typical
  anti-reflection coating is depicted in Figure 4

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Optical Properties(continued)

• Surface Texturing
• Surface texturing, either in combination with an
  anti-reflection coating or by itself, can also be
  used to minimize reflection.
• Surface texturing can be accomplished in a number
  of ways
• A single crystalline substrate can be textured
  by etching along the faces of the crystal planes.
  (random pyramid)
• the pyramids are etched down into the silicon
  surface rather than etched pointing upwards from
  the surface (inverted pyramid)
• using a photolithographic technique as well as
  mechanically sculpting the front surface using
  dicing saws or lasers to cut the surface into an
  appropriate shape (multicrystalline wafers).
• Figure 5 is shown the surface texturing which are
  used those methods

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Optical Properties(continued)

• Material Thickness
• The amount of light absorbed depends on the
  optical path length and the absorption
  coefficient.
• For silicon material in excess of 10 mm thick,
  essentially all the light with energy above the
  band gap is absorbed. The 100 of the total
  current refers to the fact that at 10 mm, all the
  light which can be absorbed in silicon, is
  absorbed.
• In material of 10 microns thick, only 30 of the
  total available current is absorbed. The photons
  which are lost are the orange and red photons.

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Optical Properties(continued)

• Light Trapping
• a solar cell with no light trapping features may
  have an optical path length of one device
  thickness, while a solar cell with good light
  trapping may have an optical path length of 50,
  indicating that light bounces back and forth
  within the cell many times.
• Light trapping is usually achieved by changing
  the angle at which light travels in the solar
  cell by having it be incident on an angled
  surface.
• the angle at which light enters the solar cell
  (the angle of refracted light) can be calculated

(6.2)
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Optical Properties(continued)

• In a textured single crystalline solar cell, the
  presence of crystallographic planes make the
  angle q1 equal to 36 as shown in Figure 6.
• Lambertian Rear Reflectors
• A Lambertian back reflector is a special type of
  rear reflector which randomizes the direction of
  the reflected light.
• A Lambertian rear surface is illustrated in the
  figure 7.

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Reducing Recombination

• Recombination Losses
• Recombination losses effect
• the current collection (the short-circuit
  current)
• the forward bias injection current (open-circuit
  voltage).
• The main areas of recombination are
• at the surface (surface recombination)
• the bulk of the solar cell (bulk recombination)
• The depletion region is another area in which
  recombination can occur (depletion region
  recombination).

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Reducing Recombination (continued)

• Current Losses Due to Recombination
• In order for the p-n junction to be able to
  collect all of the light-generated carriers, both
  surface and bulk recombination must be minimized.
  
• In silicon solar cells, the two conditions
  commonly required for such current collection
  are
• the carrier must be generated within a diffusion
  length of the junction, so that it will be able
  to diffuse to the junction before recombining
  and
• in the case of a localized high recombination
  site, the carrier must be generated closer to the
  junction than to the recombination site. For less
  severe localized recombination sites, carriers
  can be generated closer to the recombination site
  while still being able to diffuse to the junction
  and be collected without recombining.

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Reducing Recombination (continued)

• The quantum efficiency of a solar cell quantifies
  the effect of recombination on the light
  generation current. The quantum efficiency of a
  silicon solar cell is shown in Figure 8.
• Figure 9 is illustrated Quantum efficiency curves
  for three different types of crystalline silicon
  solar cells.
• Voltage Losses Due to Recombination
• The open-circuit voltage is the voltage at which
  the forward bias diffusion current is exactly
  equal to the short circuit current.
• The forward bias diffusion current is dependent
  on the amount recombination in a p-n junction and
  increasing the recombination increases the
  forward bias current.

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Reducing Recombination (continued)

• high recombination ? ? the forward bias diffusion
  current ?, which in turn reduces the open-circuit
  voltage.
• The recombination is controlled by the number of
  minority carriers at the junction edge, how fast
  they move away from the junction and how quickly
  they recombine.
• Consequently, the dark forward bias current, an
  hence the open-circuit voltage is affected by the
  following parameters
• the number of minority carriers at the junction
  edge. Minimizing the equilibrium minority carrier
  concentration reduces recombination. Minimizing
  the equilibrium carrier concentration is achieved
  by increasing the doping
• the diffusion length in the material. The
  diffusion length depends on the types of
  material. High doping reduces the diffusion
  length
• the presence of localized recombination sources
  within a diffusion length of the junction. A high
  recombination source close the the junction will
  allow carriers to move to this recombination
  source very quickly and recombine, thus
  dramatically increasing the recombination
  current. The impact of surface recombination is
  reduced by passivating the surfaces.

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Reducing Recombination (continued)

• Effect of doping (ND) on diffusion length and
  open-circuit voltage assuming well passivated
  surfaces is shown in Figure 10.
• Surface Recombination
• Surface recombination can have a major impact
  both on the short-circuit current and on the
  open-circuit voltage.
• Lowering the high top surface recombination is
  typically accomplished by reducing the number of
  dangling silicon bonds at the top surface by
  growing a "passivating" layer (usually silicon
  dioxide) on the top surface.
• Techniques for reducing the impact of surface
  recombination is depicted in Figure 11

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Top Contact Design

• Series Resistance
• In addition to maximizing absorption and
  minimizing recombination, is to minimize
  parasitic resistive losses.
• Both shunt and series resistance losses decrease
  the fill factor and efficiency of a solar cell.
• A detrimentally low shunt resistance is a
  processing defect rather than a design parameter.
  However, the series resistance, controlled by the
  top contact design and emitter resistance, needs
  to be carefully designed for each type and size
  of solar cell structure in order to optimize
  solar cell efficiency.
• The series resistance of a solar cell consists of
  several components as shown in Figure 12

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Top Contact Design (continued)

• Base Resistance
• The resistance and current of the base is assumed
  to be constant.
• The resistance to the current of the bulk
  component of the cell, or the "bulk resistance",
  Rb, is defined as

..(6.3)
taking into account the thickness of the
material. Where L length of conducting
(resistive) path rb "bulk resistivity" (inverse
of conductivity) of the bulk cell material
(0.5 - 5.0 W cm for a typical silicon solar
cell) A cell area,and w width of bulk region
of cell.
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Top Contact Design (continued)

• Sheet Resistivity
• The "sheet resistivity", which depends on both
  the resistivity and the thickness.
• For a uniformly doped layer, the sheet resistance
  is defined as

..(6.4)
where r is the resistivity of the layer and t
is the thickness of the layer. The sheet
resistivity is normally expressed as ohms/square
or W/
For non-uniformly doped n-type layers, ie., if r
is non-uniform
..(6.5)
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Top Contact Design(continued)

• Emitter Resistance
• Based on the sheet resistivity, the power loss
  due to the emitter resistance can be calculated
  as a function of finger spacing in the top
  contact.
• Idealized current flow from point of generation
  to external contact in a solar cell is shown in
  Figure 13.

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Top Contact Design (continued)

• Contact Resistance
• Contact resistance losses occur at the interface
  between the silicon solar cell and the metal
  contact.
• To keep top contact losses low, the top N-layer
  must be as heavily doped as possible.
• Figure 14 shows points of contact resistance
  losses at interface between grid lines and
  semiconductor.

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Top Contact Design (continued)

• Metal Grid Pattern
• The design of the top contact involves not only
  the minimization of the finger and busbar
  resistance, but the overall reduction of losses
  associated with the top contact.
• These include resistive losses in the emitter,
  resistive losses in the metal top contact and
  shading losses.
• The critical features of the top contact design
  which determine how the magnitude of these losses
  are
• the finger and busbar spacing,
• the metal height-to-width aspect ratio,
• the minimum metal line width and
• the resistivity of the metal.
• These are shown in the figure 15.

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Top Contact Design (continued)

• Design Rules
• for practical reasons most top surface
  metalization patterns are relatively simple and
  highly symmetrical.
• A symmetrical contacting scheme can be broken
  down into unit cells and several broad design
  rules can be determined. It can be shown
  (Serreze, 1978) that
• the optimum width of the busbar, WB, occurs when
  the resistive loss in the busbar equals its
  shadowing loss
• a tapered busbar has lower losses than a busbar
  of constant width and
• the smaller the unit cell, the smaller finger
  width, WF , and the smaller the finger spacings,
  S, the lower the losses.

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Solar Cell Structure

• Silicon Solar Cell Parameters
• For silicon solar cells, the basic design
  constraints on
• surface reflection,
• carrier collection,
• recombination and
• parasitic resistances
• The result in an optimum device of about 25
  theoretical efficiency. A schematic of such an
  optimum device is shown in Figure 16.

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Solar Cell Structure(continued)

• Basic Cell Design Compromises
• Substrate Material (usually silicon)
• Bulk crystalline silicon dominates the current
  photovoltaic market, in part due to the
  prominence of silicon in the integrated circuit
  market.
• Cell Thickness (100-500 µm)
• An optimum silicon solar cell with light
  trapping and very good surface passivation is
  about 100 µm thick.
• Doping of Base (1 Wcm)
• A higher base doping leads to a higher Voc and
  lower resistance, but higher levels of doping
  result in damage to the crystal.

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Solar Cell Structure(continued)

• Reflection Control (front surface typically
  textured)
• The front surface is textured to increase the
  amount of light coupled into the cell.
• Emitter Dopant (n-type)
• N-type silicon has a higher surface quality than
  p-type silicon so it is placed at the front of
  the cell where most of the light is absorbed.
  Thus the top of the cell is the negative terminal
  and the rear of the cell is the positive
  terminal.
• Emitter Thickness (lt1mm)
• A large fraction of light is absorbed close to
  the front surface. By making the front layer very
  thin, a large fraction of the carriers generated
  by the incoming light are created within a
  diffusion length of the p-n junction.

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Solar Cell Structure(continued)

• Doping Level of Emitter (100 W/ )
• The front junction is doped to a level
  sufficient to conduct away the generated
  electricity without resistive looses. However,
  excessive levels of doping reduces the material's
  quality to the extent that carriers recombine
  before reaching the junction.
• Grid Pattern (fingers 20 to 200mm width, placed
  1 5 mm apart)
• The resistivity of silicon is too low to conduct
  away all the current generated, so a lower
  resistivity metal grid is placed on the surface
  to conduct away the current. The metal grid
  shades the cell from the incoming light so there
  is a compromise between light collection and
  resistance of the metal grid.
• Rear Contact.
• The rear contact is much less important than the
  front contact since it is much further away from
  the junction and does not need to be transparent.
  The design of the rear contact is becoming
  increasingly important as overall efficiency
  increases and the cells become thinner.

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Thank You
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Figure 1. Evolution of silicon solar cell
efficiency.
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Figure 2. Sources of optical loss in a solar
cell.
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Figure 3. Use of a quarter wavelength
anti-reflection coating to counter surface
reflection.
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Figure 4. Comparison of surface reflection from a
silicon solar cell, with and without a typical
anti-reflection coating.
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Figure 5. (a) A square based pyramid which forms
the surface of an appropriately textured
crystalline silicon solar cell.(b)Scanning
electron microscope photograph of a textured
silicon surface.(c) Scanning electron microscope
photograph of a textured silicon surface. (d)
Scanning electron microscope photograph of a
textured multicrystalline silicon surface.
(b)
(a)
(d)
(c)
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Figure 6. Reflection and transmission of light
for a textured silicon solar cell.
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Figure 7. Light trapping using a randomized
reflector on the rear of the cell. Light less
than the critical angle escapes the cell but
light greater than the critical angle is totally
internally reflected inside the cell. In actual
devices, the front surface is also textured using
schemes such as the random pyramids mentioned
earlier.
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Figure 8. Typical quantum efficiency in an ideal
and actual solar cell, illustrating the impact of
optical and recombination losses.
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Figure 9. Quantum efficiency curves for three
different types of crystalline silicon solar
cells. The buried contact and screen printed
curves are internal quantum efficiencies, while
the PERL is an external quantum efficiency. The
PERL cell has the best response to infrared light
since it has a well passivated, highly reflective
rear incorporating light trapping.
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Figure 10. Effect of doping (ND) on diffusion
length and open-circuit voltage assuming well
passivated surfaces.
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Figure 11. Techniques for reducing the impact of
surface recombination.
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Figure 12. Resistive components and current flows
in a solar cell.
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Figure 13. Idealised current flow from point of
generation to external contact in a solar cell.
The emitter is typically much thinner than shown
in the diagram.
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Figure 14. Points of contact resistance losses at
interface between grid lines and semiconductor.
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Figure 15. Key features of a top surface
contacting scheme.
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Figure 16. Basic schematic of a silicon solar
cell. The top layer is referred to as the emitter
and the bulk material is referred to as the base.